What ways are there to disrupt spellcasting in 5e?

[Rukelnikov]

snip

Too long to answer each individually, plus, I have already covered most (if not all) of them on different posts.

But you just said a starts finishes as soon as it starts.

So if the reaction is as soon as the trigger finishes, and the starts finishes as soon as it starts, it means that according to you, the reaction is as soon as the trigger starts.

Is that correct? If no, could you explain how it is not?

The starting of something is instantaneous "when the chronometer starts counting", first it wasn't and then it is, thus it started.

Once it is counting, it has finished beginning to count.